This invention relates to a process for the production of olefins, especially a two stage process wherein, in the first stage, heavy petroleum fractions are hydrogenated in the presence of hydrogen and a hydrogenation catalyst and, in the second stage, the thus-hydrogenated fractions are subjected to thermal cracking in the presence of steam.
Such a process has been known from DOS [German Unexamined Laid-Open Application] No. 2,164,951, and also U.S. Pat. Nos. 3,720,729 of Sze et al, Mar. 13, 1973 and 3,781,195 of Davis et al, Dec. 25, 1973. In contrast to a direct thermal cracking of heavy crude oil fractions, wherein a great amount of pyrolysis oil, tar, and coke is produced in addition to a minor yield, it is possible by means of the conventional method to produce, by catalytic hydrogenation of the crude oil fractions prior to thermal cracking, hydrogenated fractions which are suitable for thermal cracking.
The heavy petroleum fractions fed to the first stage contain a high proportion of aromatic and heterocyclic compounds. A disadvantage in the conventional process is that decomposition reactions also occur due to the acidic properties of the catalyst supports. These decomposition products in addition consume more hydrogen which otherwise would be unnecessary in this process stage.